Check for unused registered variables

[gardar]

It would be helpful to have a check that verifies that the variable set by register in a task is actually being used in any of the following tasks and warns if it's unused.

[hoo29]

Now implemented in little-timmy 2.0.1, an unused ansible variable finder
https://github.com/hoo29/little-timmy
https://pypi.org/project/little-timmy/